1. Yes, there are solutions with only one card now. They go in the slot where NDS games go. They are newer than the slot-2 (GBA slot) solutions that required a slot-2 card and a passme or nopass in the NDS slot, and have some significant advantages. Also, what do you mean by "old brick DS"?

2. Yes, many ROMs can be saved on a card. For a 1GB microSD card usually about 20-30 can be saved but for cards like a 4GB microSDHC >100 can be saved.

RHS is an excellent retailer and is highly trusted, yes. Check ResellerRatings if you'd like to see what I mean. Focus more on the reviews themselves, as the score was lowered by two people who went overboard with their dissatisfaction, blaming the shop for things beyond its control.